1. Field of the Invention
The present invention relates generally to the field of geophysics. More specifically, the present invention discloses a method for converting seismic data from the time domain to the depth domain.
2. Statement of the Problem
Seismic data are widely used in petroleum and mineral exploration and development, as they provide detailed areal information used in planning exploration and development activities. Seismic data are recorded and evenly sampled in time—the time required for the energy to travel to the geophone or hydrophone from the initiation of the energy source. Values of seismic attributes, such as acoustic or elastic impedance, can be useful in building or constraining geologic models of rock properties. However, these rock property models are constructed in depth, not in time. The conversion from time to depth is non-trivial, as lateral variations in velocity and reflection static values add local complexity to the conversion from time to depth. These local complexities are not captured by smoothly varying velocity functions (or constant velocity functions) typically used in the time-depth conversion process.
In addition, log data recorded in wells are used to obtain information on the structure and physical attributes of the subsurface. However, seismic data are recorded in time, whereas log data are recorded in depth. In order to incorporate the seismic data information with the well data when generating geologic models in depth, the seismic data must necessarily be converted to depth.
Geophysicists have long converted time maps to depth, as maps in depth are required for use in exploration and development activities. This requires multiplying the one-way travel time obtained from the seismic data at each seismic trace by a corresponding velocity value to obtain a depth. Various methodologies, involving average and/or interval velocity functions, are used to convert the time maps to depth maps. However, converting a seismic data volume from time to depth requires a different procedure, as this involves a velocity-dependant resampling of the data as well as a conversion of the time axis values to depth values.
Solution to the Problem. The present invention addresses many of the shortcomings of the prior art in this field by providing a method for converting seismic data from the time domain to the depth domain that accommodates interval velocity information that can vary significantly from sample to sample in both lateral (spatial) and vertical (temporal) directions if necessary. This approach provides significantly improved accuracy in converting the seismic data volume to depth.
Additionally, for applications in the petroleum industry, only a selected interval, not the entire seismic time range, needs to be depth converted. The present methodology allows the user to convert a selected time range, delineated by an uppermost and lowermost seismic horizon, to be converted to depth. Additionally, the inclusion of additional intermediate horizons between the uppermost and lowermost horizons allows for handling complex stratigraphic relationships, such as truncation and onlap, where the zone does not exist everywhere within the seismic data.